This invention relates to a method for the production of succinimide copolymers by the catalytic co-polymerization of aspartic acid with another monomer.
Polysuccinimides, polyaspartates and copolymers thereof are useful as mineral scale inhibiting agents, nutrient absorption enhancers, additives for cosmetics and personal care products, adhesives, anti-redeposition agents for detergents, dispersants, additives for paper making, corrosion inhibitors, metal working fluids, lubricants for conveyor belts, additives for the prevention of encrustation in sugar manufacture, and tartar preventative agents in toothpaste.
Methods are known for the production of polysuccinimide by the polymerization of aspartic acid in the presence of various catalysts, such as phosphoric acid and sulfur-containing dehydrating agents and the like. However, these prior art methods rely on an inefficient process of heat transfer during polymerization, namely the heating of an unstirrable melt. Another disadvantage to these methods is the large amount of catalyst and/or dehydrating agent which is required for polymerization to occur and the subsequent removal of the excess catalyst.
Attempts to perform catalytic polymerizations as stirrable, liquid reactions have been made, but again exceedingly high amounts of catalyst were required to achieve the polymerization of desirable high molecular weight products (U.S. Pat. No. 5,484,945 to Nagatomo et al.).
There is an ongoing need therefore, for a commercially acceptable, convenient method of catalytically polymerizing aspartic acid to polysuccinimide and succinimide copolymers in high yield, purity and of desired, relatively high molecular weight. The present inventive method satisfies this need, provides a useful product and overcomes the disadvantages of the prior art methods.
An efficient solution-phase method of succinimide copolymer production is disclosed. A liquid reaction mixture containing at least one cyclic carbonate solvent, at least one catalyst, and aspartic acid, optionally together with another polymerizable monomer, is initially prepared. Preferably, the weight ratio of aspartic acid/catalyst is greater than about 1.
The resulting reaction mixture is heated to an elevated temperature which is below the boiling point of the solvent but is sufficient to effect the catalytic polymerization of the aspartic acid in solution. Thereafter, the temperature is maintained for a sufficient time period until a succinimide copolymer with another polymerizable monomer is produced. Succinimide copolymers of relatively high weight average molecular weight and high purity can be produced in relatively high yields even when relatively low ratios of catalyst are employed.
The succinimide copolymers so produced can be recovered by precipitation with a triturating solvent, and hydrolyzed to a polyaspartic acid derivative if desired.
The inventive process requires only a single, stirred, reactor vessel thereby avoiding the problems of prior multi-step methods with handling and recovering the product from semi-solid melts. The inventive method provides a succinimide copolymer in relatively high yields and of relatively high purity.
Aspartic acid and at least one catalyst can be dissolved in at least one cyclic carbonate solvent and can be co-polymerized with another monomer, as well as homopolymerized with itself, in solution by the application of heat, the temperature being maintained below the boiling point of the solvent. The aspartic acid can be in any of its L-, D-, and DL-isomer forms. Relatively low catalyst loadings can be used. The term xe2x80x9crelatively low catalyst loadingsxe2x80x9d as used herein means that the ratio of aspartic acid/total catalyst on a solids weight/weight (w/w) basis is greater than about 1.
As presently practiced, the inventive method can produce a succinimide copolymer having a weight average molecular weight (Mw) in the range of about 700 to about 100,000. The molecular weight can be controlled by varying one or more of the following reaction conditions: co-monomer, solvent, concentration of reactants, polymerization temperature, polymerization time, reaction pressure, water removal rate, catalyst, and weight ratio of aspartic acid monomer to catalyst. The term xe2x80x9csuccinimide copolymerxe2x80x9d as used herein and in the appended claims includes polysuccinimide copolymers with another monomer moiety as well as random copolymers constituted by succinimide units with another monomer.
Cyclic carbonate solvents useful in the inventive method preferably have a boiling point in the range of about 150xc2x0 C. to about 300xc2x0 C. Cyclic carbonates presently include cyclic organic esters having the formula: 
wherein R1, R2, R3 and R4 are independently hydrogen, or alkyl (1 to 20 carbon atoms inclusive), aryl, or hydroxymethyl or chloromethyl.
In a particularly preferred method embodiment, the cyclic carbonate solvent is unreactive with respect to aspartic acid and the monomer to be copolymerized, can solubilize the product succinimide copolymer, and is commercially available at reasonable cost.
Presently preferred solvents are cyclic alkylene carbonates. Examples include ethylene carbonate, propylene carbonate, butylene carbonate, glycerin carbonate, and mixtures thereof. Many such cyclic carbonates are commercially sold under the trademark JEFFSOL(copyright) by Huntsman Corporation, Austin, Texas.
The reaction mixture can be formed by combining in a cyclic carbonate solvent, aspartic acid and at least one desired co-polymerizable monomer in the presence of a polymerization catalyst. The copolymerizable monomer has a functionality of at least 2. Suitable monomers for the present purposes are those that are soluble in the cyclic carbonate solvent that is utilized as the reaction medium in any given instance and that possess. the desired polyfunctionality. Preferred monomers are poly(carboxylic acids), aminocarboxylic acids, mercaptocarboxylic acids, sulfocarboxylic acids, phosphonocarboxylic acids, phosphinocarboxylic acids, hydroxy-carboxylic acids, diamines and triamines. Mixtures of the aforementioned polyfunctional monomers can also be used to produce succinimide copolymers having properties tailored for a particular purpose.
The order of addition is not important so long as a substantially liquid reaction mixture is obtained with heating. The temperature must be sufficiently high to initiate polymerization of the aspartic acid and will vary with operating conditions. Preferably, the elevated temperature remains below the boiling point of the chosen cyclic carbonate solvent. The elevated temperature is in the range of about 140xc2x0 C. to about 220xc2x0 C. Preferably the reactants remain in solution during the whole course of the polymerization reaction. Reaction times can vary in the range of about 5 minutes to about 24 hours, preferably about 30 minutes to about 12 hours.
Succinimide copolymer product is obtained in relatively high yields of about 40% to about 100% and with relatively high purity of about 70% to about 100%.
The reaction pressure can be atmospheric (air or inert gas) or sub-atmospheric. The gas is preferably anhydrous nitrogen, or carbon dioxide and can be passed through the reactor.
The succinimide copolymer product can be linear or branched, can be a random copolymer as well as a block copolymer or a graft linear copolymer, and is recovered by precipitation through the addition of a triturating solvent which is miscible with the cyclic carbonate solvent but is non-solvating for the polysuccinimide product. Useful triturating solvents include, but are not limited to, ketones, alcohols, esters, nitrites, water, and hydrocarbons. Acetone is particularly preferred. The succinimide copolymer product can also be isolated by other solvent separation techniques, such as flash evaporation or distillation.
The succinimide copolymers produced by the present inventive methods can be used directly or can be hydrolyzed to produce corresponding polyaspartate derivatives.
Catalysts useful for the aspartic acid co-polymerizations include, without being limited to, known phosphorus-containing catalysts, sulfur- and oxygen-containing dehydrating agents and mixtures thereof and catalysts disclosed in U.S. Pat. No. 5,508,434 (Batzel et al.), incorporated herein by reference.
Examples of phosphorus-containing catalysts include phosphoric acid, polyphosphoric acid, phosphorous acid, and hypophosphorous acid, as well as mixtures of the foregoing. Examples of sulfur- and oxygen-containing dehydrating agents include sulfur trioxide anhydride and sulfur trioxide precursors, complexes of sulfur trioxide with amines or amides, alkyl sulfonic acids, aryl sulfonic acids, alkali, alkaline earth, and amine salts of alkyl sulfonic acids, aryl sulfonic acids, anhydrosulfuric acids and salts thereof, sulfurous acid, and alkali, alkaline earth, or amine salts of sulfurous acid.
Particularly preferred sulfur trioxide precursors include sulfur oxygen acids, organic amine salts and inorganic salts of sulfur oxygen acids, coordinations complexes of sulfur trioxide and aliphatic amines or heterocyclic amines, complexes of sulfur trioxide and water-miscible aprotic solvents and mixtures thereof.
Preferred sulfur oxygen acids are sulfuric acid, fuming sulfuric acid, polysulfuric acid, and inorganic or organic salts and mixtures thereof.
Mixtures of more than one catalyst may be used in the methods of the present invention. The use of such mixtures of catalysts may be advantageous for economic reasons and for the production of desired molecular weight succinimide copolymers.
Suitable co-monomers with aspartic acid are saturated as well as unsaturated poly(carboxylic acids), hydroxycarboxylic acids, aminocarboxylic acids including aminoalkylcarboxylic acids as well as aminoaryl carboxylic acids, mercaptocarboxylic acids, sulfocarboxylic acids, phosphonocarboxylic acids, phosphinocarboxylic acids and organic diamines or triamines, as long as the selected co-monomer is soluble in the solvent that is utilized as the copolymerization medium.
Organic primary or secondary amines can be utilized for end-capping, i.e., chain termination, purposes and also to graft a pendant chain onto the produced polymer backbone.
Illustrative saturated poly(carboxylic acids) are adipic acid, 1,2,3,4-butanetetracarboxylic acid, decanedioic acid, pentane-1,3,5-tricarboxylic acid, phthalic acid, and the like. Illustrative unsaturated poly(carboxylic acids) are maleic acid, fumaric acid, aconitic acid, and the like.
Among suitable substituted carboxylic acids illustrative are hydroxycarboxylic acids such as lactic acid, citric acid, tartaric acid, and the like. Illustrative aminocarboxylic acids are the naturally-occurring xcex1-amino acids such as glutamic acid, and the like, amino acid dimers such as cystine 
lanthionine (Ala-Cys), cystathionine (Ala-Hcy), and the like, aminoalkylcarboxylic acids such as aminobutyric acid, and the like, aminoarylcarboxylic acids such as aminobenzoic acid, and the like. Illustrative mercaptocarboxylic acids are 3-sulfanylpropanoic acid, and the like, as well as homocysteine, and the like. Illustrative sulfocarboxylic acids are 5-sulfosalicylic acid, 5-sulfoisophthalic acid, 4-sulfophthalic acid, sulfosuccinic acid, and the like. Illustrative phosphonocarboxylic acids are phosphonosuccinic acid, 2-phosphorus-1,2,4-butanetricarboxylic acid (commercially available under the designation xe2x80x9cBayhibitxe2x80x9d from Bayer Corporation, Elkhart, IN, U.S.A.), D,L-2-amino-4-phosphonopropionic acid, and the like. Illustrative phosphinocarboxylic acids are phosphinosuccinic acid, and the like.
Relatively low catalyst loadings were found to produce succinimide copolymers of relatively high weight average molecular weight (Mw). The ratio of aspartic acid/catalyst on a solids basis varied in a range of from about 1/1 w/w to about 40/1 w/w. The specific ratio employed varied depending on the operating conditions and Mw desired as described in the Examples listed below. A preferred aspartic acid/catalyst ratio is from about 3.5/1 w/w to about 20/1 w/w.
The present inventive method advantageously employs a one-pot reaction. The reactants are added to the cyclic carbonate solvent forming a mixture which is then heated to dissolve the reactants and to initiate the polymerization reaction. The reaction can be carried out in a conventional heated and stirred reactor. Such reactors are relatively inexpensive, commonly available, and allow high production rates in small plant areas. Recovery of the succinimide copolymer can be accomplished by employing simple precipitation or like techniques.
During the polymerization reaction, the water of condensation preferably is removed by employing a condenser. The use of reduced pressure, about xe2x88x9288 kPa, during the polymerization reaction allows for faster removal of water, thereby promoting faster reactions resulting in higher molecular weight products.
The use of solution polymerization in the inventive method allows a much higher degree of control in the selection of the molecular weight of the product. Since low amounts of catalyst can be used, the use of more expensive, efficient catalysts is permitted without significant cost increases. The consistency and homogeneity of the reaction product is also greatly improved. As presently practiced, the inventive method produced succinimide copolymers of very low to no color. Polyaspartate derivatives derived therefrom by hydrolysis were also generally of low color.
In the practice of the inventive method, the reaction mixture can be formed by initially adding the aspartic acid and the co-monomer to a mixture of solvent and catalyst, then heating the resulting reaction mixture. In addition, the catalyst can be added to a mixture of the solvent, aspartic acid and co-monomer; or co-monomer may be added to a mixture of aspartic acid, catalyst and solvent. The solvent may optionally be preheated prior to addition of the aspartic acid, co-monomer or catalyst.
The following Examples employ generally preferred materials to further illustrate the inventive method but are not intended to be limiting. In each of the Examples, L-aspartic acid (Nanjing Jinke, China) was employed. The formation of polysuccinimide was confirmed by Infrared Spectroscopy (IR) analysis. Unless indicated otherwise, the solid reaction product was collected by filtration, weighed, and the yield as a percentage of theoretical yield was calculated.
The molecular weight of the polysuccinimide or its derivatives produced was determined by base hydrolysis of the polysuccinimide with aqueous sodium hydroxide forming the sodium salt of polyaspartic acid. The number average molecular weight (Mn), weight average molecular weight (Mw), and the Z-average molecular weight (Mz) of the sodium polyaspartate were determined by Gel Permeation Chromatography (GPC) analysis measured with reference to polyacrylate standards. The percent aspartic monomer was determined by titration with perchloric acid in a mixed solvent of acetic acid and formic acid (reference: Amino Acids and Related Compounds. Specification/General Tests; Kyowa Hakko; 3rd Ed., Kogyo Co., Tokyo, Japan, pg. 88-89). The purity of the polysuccinimide products was determined by subtracting the monomer content from about 100%. Lack of excess crosslinking in the polymer product was determined by NMR spectroscopy.